Pigment dispersions are part of many systems and compositions. For example, even in only the printing and imaging field, pigment dispersions are used in making photoreceptors, inks, toners, paints, and the like. Generally, pigmented systems require the grinding down of large-sized pigment agglomerates to desired small-sized pigment sized dispersions, such as grinding down from micron-sized agglomerates to desired nanometer-sized particles. The pigments are also generally contained in a dispersion, where the grinding down or particle size reduction is accompanied by appropriate dispersants to achieve stable dispersions.
The most common method for pigment particle size reduction is to use milling media to accomplish the particle size reduction. Historically, this milling has occurred with the use of millimeter- to sub-millimeter-sized media, such as in the range of 6 or 3 mm down to about 600 μm. Recently, the trend has been to further decrease the milling media size, such as to about 300 μm. The smaller media particle size is desired because the smaller the milling media particle, the faster the particle size reduction of the pigment agglomerates, although with an appropriate increase in milling media agitation.
A problem with reducing the milling media particle size, however, is that generally as the particle size decreases, the product cost of the milling media increases. While it may be desirable to decrease the milling media particle size below 300 μm, that smaller size milling media becomes prohibitively more expensive. For example, the product cost for milling media can reach or exceed $1,000/kg for sub-100 micron beads.
Numerous processes are within the purview of those skilled in the art for forming a wide variety of compositions using pigments and pigment dispersions. For example, inks can be formed by mixing the pigment dispersion with a liquid carrier and other optional components; photoreceptors can be formed by mixing the pigment dispersion with a film-forming resin to form different layers of the photoreceptors; and toners can be formed by mixing the pigment dispersion with resin particles, wax particles, and other additives, either in a conventional mixing apparatus or in en emulsion/aggregation process.
It would be advantageous to provide new and improved processes for forming pigment dispersions of a desired small particle size, using less expensive milling media and milling processes.